It all depends on how efficient you are at packing, Mathieu. I mean, rather obviously you won't be taking clean clothing for all three months. Nor will you be able to really carry around much that you buy; you'll probably just ship things home. So, how many days' worth of stuff do you need at one time? Three? Five? Seven?
In most cases, I think a pack with anywhere from 4,000 to 4,500 cubic inches (about 68 to 73 liters) should be adequate. That should be enough room for several changes of clothing, toiletries, maybe spare shoes. A real solid choice would be Osprey's Farpoint 70 ($200). It toes the 70-liter line, which would force you to be efficient but that is not a bad thing. Otherwise, it's a great travel pack, very sleek, stowable suspension, side handles for easy lugging, lots of packing options.
Up your way, Mountain Equipment Co-op sells its own brand of travel pack called the Pangea 75 ($219 Canadian). A bit bigger than the Osprey pack, right at 75 liters. It has a detachable daypack which may be useful, and lots of internal organizing options as well as external attachment points. Like the Osprey it has a stowable suspension.
One other option is to go with a bag that has both rollers and backpack straps. These typically don't make as good a backpack as a model built specifically for that purpose. But heaven knows pulling a pack behind you on rollers is a lot easier than carrying it on your back. Eagle Creek makes a really nice hybrid backpack/roller bag, the Switchback Max 25 Pack ($350 U.S.). It has a full stowable suspension—shoulder straps and hip belt—plus rollers and a retractable handle. It's a 75-liter pack, with a small detachable daypack. Eagle Creek really knows how to organize a pack, so the Meridian has lots of packing options and is designed to carry Eagle Creek's very good line of packing accessories (sleeves for shirts and so on).
Have a great trip!